A year ago, March 21 was the coolest day in a 5-day stretch. Roanoke’s high was 73 — two days before and two days after were in the 80s. Roanoke’s low on March 21, 2012, was 54 — compared to a normal high of 59. It doesn’t get much warmer than that to start astronomical spring. But a year later, Thursday will be among the coldest days Southwest Virginia has ever seen once the calendar has officially flipped to spring. The NOAA map at left shows a projected high of 29 degrees at Blacksburg. If that were to verify — it may not, even if the afternoon is that cold, since it’s possible the early morning temperature shortly after midnight will be warmer — it would be only the second time in the history of Blacksburg’s weather data, going back to the 1950s, that a day in astronomical spring fell short of 30 for a high (29 on March 31, 1964, was the other). These 25-degrees-below-normal high temperatures are the result of an intense “blocking” pattern of high pressure that is trapping extremely cold air, relative to the season, across central and eastern North America. It shows up in the initiation frame of the European forecast model from this morning — the green colors are high pressure over Greenland and the North Pole, with the blue colors trapped to the west and south. This blocking pattern will only slowly ease in days ahead — some of the storm systems moving through will actually only add to it, becoming a log jam behind the strong high over Greenland. So it appears below-normal temperatures will be the rule for at least the next 10 days, perhaps even 2 weeks or more, reaching early April.
You’ll probably see a few snowflakes flying through the air in the Roanoke and New River valleys and farther west from time to time on Thursday, as is often the case when strong northwest winds blow up and over the Appalachians, lifting and condensing moisture in the cold air just above the surface. Enough snow to briefly turn the ground white may occur in a few spots in the New River Valley, and many of the usual places that get whitened in upslope snow situations (west of Interstate 77, near the Virginia-West Virginia border) will be again.
That brings us back to the weekend and the chance for a Palm Sunday snowfall. A storm system taking shape in the south-central U.S. will move east over the weekend. How exactly it moves and evolves, and how much cold air remains banked against the mountains when it approaches, will determine the proportion of snow to sleet to rain that we see in the general time frame of late Saturday night to Monday. The inland low will eventually transfer energy to a coastal low, but the bulk of any snow we get in Southwest Virginia may be determined by how much moisture from the inland low can be squeezed out in below-freezing air before it warms slightly in the layers above the surface. Generally speaking a more southerly track to the low, more intense cold air wedging from the northeast and a more southerly handoff of energy from the inland low to the coastal will favor more snow or snow/sleet mix for our region rather than rain. Forecast models will be slowly be fed greater and more relevant data the next couple of days and will hopefully pinpoint some of these attributes a little better. Best general idea now is to expect some snow, gradually becoming mixed with and/or changing to sleet and rain, sometime in the time frame from late Saturday night to Monday. But much is still in flux, as it always is more than 48 hours from a potential storm. The pieces are on the board that could be put together into the largest widespread late March snow we’ve seen in nine years. Stay tuned.